Author Topic: Trace Inverter - Low output voltage  (Read 6347 times)

Offline gus

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Re: Trace Inverter - Low output voltage
« Reply #15 on: June 15, 2011, 01:43:18 PM »
I have an ancient Sears analog VOM that has none of these RMS problems and I haul it out when my digitals give me fits.

It even has a mirror behind the needle, remember those?
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Offline bevans6

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Re: Trace Inverter - Low output voltage
« Reply #16 on: June 15, 2011, 02:32:21 PM »
To allow you to eliminate parallax errors...

one thing I learned back then was the difference between accuracy and precision.  We had to use slide rules for tests, and while they allowed calculators if you gave and answer that had more significant digits than you could get with a slide rule you got marked down...
1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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Tatamagouche, Nova Scotia

Offline Sean

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Re: Trace Inverter - Low output voltage
« Reply #17 on: June 15, 2011, 02:47:59 PM »
Hmm... I feel an "Electrically Speaking" column coming on.

Folks, the only way you can get a "correct" reading of the output of this inverter, or almost any MSW (or square-wave) inverter, is with a "true RMS" meter.  There is no multiplication factor or correction you can apply to a standard non-RMS meter, nor will a non-RMS meter with an old-fashioned analog movement help.

Now, once you know the correction for any given inverter, then, yes, you can use a standard meter to check that inverter later, assuming the waveform does not change (not necessarily a valid assumption for all models).  But you can not derive that correction from, say, the relationship between peak and RMS voltage on a perfect sine wave.  It doesn't depend on the relationship in a real sine wave, it depends on the relationship in the waveform actually produced, which will vary from waveform to waveform (and, in some cases, even moment to moment).

Induction motors, BTW, should NOT be run on inverters unless they are "true sine" models.  I've gone into great detail on why this is elsewhere on the board, so will not repeat it here, other than to say that Brian's CEMF explanation is related.  Bottom line is that the inductance in the stator will oppose the too-rapid change in voltage, heating the stator windings.  You'll expend more power for less work, generate heat, and possibly burn out the motor windings.  (Connecting a large inductance such as a motor to the output will not necessarily improve your non-RMS meter's readings, though.)

-Sean
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